In the past two decades there has been an increasing interest in the use of lasers in material processing applications. Recent developments of pulsed laser technology have been lasers producing laser pulses of extreme high intensity—in the order of Giga Watts—combined with a very short pulse length—in the order of femtoseconds. The combination of high pulse intensity and very short pulse length allows very precise and localized material processing. The intensity is high enough to cause physical effects in a volume element encompassing the focus of the laser beam resulting in permanent changes of the affected material or ablation at the surface. However, the pulse length is short enough to limit the energy transmitted in a pulse such that the material in the vicinity of the focus is able to absorb the same without suffering serious damage. Applications of femtosecond laser pulses in material processing are disclosed in U.S. Pat. No. 5,656,186 issued to Mourou et al. in Aug. 12, 1997, and in U.S. Pat. No. 6,156,030 issued to Neev in Dec. 5, 2000.
Recently, it has been recognized that femtosecond laser pulses allow modifying the refractive index inside dielectric materials on a microscopic scale. This leads to the possibility of writing optical structures such as waveguides inside transparent materials as shown in:
K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, Opt. Lett. 21, 21 (1996);
K. Miura, H. Inouye, J. Qiu, T. Mitsuyu, K. Hirao, NIM B, 141 (1998);
K. Hirao and K. Miura, J. Non-Crys. Solids 91, 235 (1998);
S. H. Cho, H. Kumagai, K. Midorikawa, M. Obara, SPIE'99, SPIE Vol. 3618;
C. B. Schaffer, A. Brodeur, J. F. Garcia and E. Mazur, Opt. Lett. 93, 26 (2000);
D. Homoelle, S. Wielandy, A. I. Gaeta, N. F. Borrelli and C. Smith, Opt. Lett. 1311, 24 (1999);
L. Sudrie, M. Franco, B. Prade and A. Mysyrowicz, Opt. Comm. 279, 171 (1999); and,
A. Yu. Naumov, C. Przygodzki, X. Zhu, P. B. Corkum, CLEO'99, CThC2, p356.
It is generally assumed that in the paraxial limit laser beams with a power greater than the self-focusing power must always self-focus and that the loss of control over beam propagation resulting from self-focusing make controlled energy deposition through multiphoton ionization difficult, if not impossible. Numerous articles have been published considering this phenomenon:
A. Yariv, Quantum Electronics (Wiley, New York, 1975);
Y. R. Shen, Prog. Quantum Electron. 4, 1 (1975);
J. H. Marburger, Prog. Quantum Electron. 4, 35 (1975);
J. Ranka, R. W. Schirmer, and A. Gaeta, Phys. Rev. Lett. 77, 3783 (1996);
J. F. Lami, S. Petit, and C. Hirlimann, Phys. Rev. Lett. 82, 1032 (1999);
A. Zozulya, S. Diddams, A. V. Engen, and T. S. Clement, Phys. Rev. Lett. 82, 1430 (1999);
Gaeta, Phys. Rev. Lett. 84, 3582 (2000);
J. Rothberg, Opt. Lett. 17, 583 (1992);
D. Strickland and P. Corkum, J. Opt. Soc. Am. B 11, 492 (1994); and,
G. G. Luther, J. V. Moloney, A. C. Newell, and E. M. Wright, Opt. Lett. 19, 862 (1994).
It would be advantageous to control the energy deposition process in Femtosecond Laser Dielectric Modification (FLDM), in particular, at intensities higher than the threshold for self-focusing in the material. This would allow achieving controlled energy deposition with high precision, which is needed to produce useable optical structures. Furthermore, it would be advantageous to have a method for modeling the plasma distribution induced through non-linear absorption of a femtosecond laser pulse and predicting the energy transmitted through the focus. This would enable reproducible manufacture of optical structures used, for example, in fiber optic networks.
It is, therefore, an object of the invention to provide a method for modeling the plasma distribution induced through non-linear absorption of a femtosecond laser pulse and predicting the energy transmitted through the focus.
It is further an object of the invention to provide methods and devices for manufacturing optical structures using FLDM.